Double pass optical system for raster scanners

ABSTRACT

A raster scanner with laser for generating a scanning beam and compact folded optical system for transmitting the beam to the object to be scanned. The optical system includes a first mirror for guiding the laser beam forward along a downwardly inclined slope to the modulator, a second mirror below the modulator for intercepting the beam from the modulator and folding the intercepted beam to guide the beam backwards along a horizontal plane to a third mirror which folds the beam and guides the beam forward along an upwardly inclined slope against the mirrored facets of a rotating polygon, the polygon scanning the beam through a preset scan arc and returning the beam via a focusing lens to the third mirror, the third mirror folding the scan beam and guiding the scan beam forward, and a fourth mirror for intercepting the scan beam and guiding the beam to the object to be scanned.

This invention relates to raster scanners, and more particularly to araster scanner incorporating a compact folded optical system.

In raster type scanners which may, for example be used to write imageson the photoconductor of a xerographic apparatus for subsequentdevelopment and transfer to a copy substrate material, typically employa laser as the source of the scanning beam. An optical system whichincludes a rotating, scanning polygon sweeps the beam across the objectbeing scanned as for example the aforementioned photoconductor. In thecase where the scanner serves to produce or write images, a modulator isdisposed astride the beam path to vary the intensity of the beam inaccordance with video image signals input thereto.

In scanners of the aforementioned type, the optical tolerances areextremely close in order to assure that a beam of the requisite size andintensity strikes the object being scanned and the correct imagecontrast, size, orientation, etc. is achieved.

However, the optical requirements needed to meet the strict opticaltolerance levels required may dictate a relatively elaborate opticalpath in order to meet the operational constraints imposed by the variousoptical components, and particularly the scanning polygon, lens, andbeam modulator. And if the original intent was to design a relativelycompact and inexpensive raster scanner, the particular dimensional andcomponent interrelationships imposed may instead result in a scanner ofa size and/or cost substantially greater than that envisioned or desiredoriginally.

The invention relates to a raster scanner having a folded optical pathto reduce size and cost. The scanner includes, in combination, a sourceof a high intensity light beam; a first mirror for intercepting the beamand reflecting the beam forward along a downwardly inclined slope; asecond mirror for intercepting the beam from the first mirror andfolding the beam so that the beam is reflected backwards along asubstantially horizontal plane; a polygon for scanning the beam througha predetermined scan arc; and a third mirror for intercepting the beamreflected from the second mirror and folding the beam so that the beamis reflected forward along an upwardly inclined slope toward thepolygon, the polygon intercepting the beam from the third mirror andreflecting the beam backward along the slope to the third mirror, thethird mirror intercepting the scan beam from the polygon and reflectingthe scan beam forward along the horizontal plane to the object to bescanned.

Other objects and advantages will be apparent from the ensuingdescription and drawings in which:

FIG. 1 is an isometric view of an exemplary laser driven raster scannerembodying the principles of the present invention;

FIG. 2 is a side view of the scanner shown in FIG. 1;

FIG. 3 is a top view of the scanner shown in FIG. 1;

FIG. 4 is an enlarged view showing details of the laser supportingmechanism enabling field replacement and realignment of the laserfollowing servicing or replacement thereof;

FIG. 5 is a block diagram of the scanner control;

FIG. 6 is a logic circuit diagram showing details of the scannerdetector circuit including a peak detection circuit enabling the scandetector to serve also as a light meter when aligning the laser;

FIGS. 7a and 7b are two parts of a logic circuit diagram showing detailsof the scanner pixel clock generator;

FIG. 8 is a logic circuit diagram showing details of the polygon motordriver; and

FIG. 9 is a logic circuit diagram showing details of the modulatordriver.

Referring to FIGS. 1-3 of the drawings, a raster output scanner (ROS) 10embodying principles of the present invention is thereshown. As willappear, scanner 10 generates latent electrostatic images on thephotoconductive surface 12 of a xerographic member 11 (shown here in theform of a drum) of a xerographic system (not shown). As will beunderstood by those familiar with the xerographic arts, the latentelectrostatic images are created on the previously uniformly chargedphotoconductive surface 12 through selective exposure thereof inresponse to image information in the form of video image signals orpixels input to modulator 27 of scanner 10. The latent electrostaticimage so created is thereafter developed and the developed imagetransferred to a suitable copy substrate material i.e. a copy sheet. Thetransferred image is thereafter fixed to form a permanent copy.

Scanner 10 includes a generally rectangular base 14 on which the severalcomponents of scanner 10 are mounted in operative relation. Base 14includes upright end supports 17, 18. A polygon bridge support 19extends downwardly from the upper portion of end support 17 to edge 14'of base 14. An oppositely facing, downwardly inclined side support 20extends along one side of base 14 from the corner area of end support 18to the opposite end of base 14 proximate end support 17. The angle ofinclination of bridge support 19 and side support 20 is chosen toaccommodate system optical requirements and assure maximum compactness.

A suitable source of high intensity light such as a laser, LightEmitting Diodes (LEDs), Infra-Red (IR) laser diodes, etc. is provided.In the exemplary arrangement shown, a laser assembly 15 with laserplasma tube or laser 22 is mounted on end support 18 in a plane spacedabove the plane of base 14. The longitudinal axis of laser assembly 15generally parallels edge 14' of base 14 and end supports 17, 18. As willappear more fully herein, laser assembly 15 is mounted on end support 18through an adjustable supporting mechanism which enables the beam oflight output by laser 22 to be aligned with the scanner optical axis inthe field by service personnel. A beam focusing lens 67 is provided tofocus the laser beam internal to modulator 27, as will appear.

A movable shutter 21 is disposed adjacent the beam discharge side oflaser assembly 15, shutter 21 serving to intercept the beam 25 emittedby laser 22 when scanner 10 is not in use. This permits laser 22 to beoperated continuously, prolonging laser life. A solenoid 23 is providedfor withdrawing shutter 21 when it is desired to operate scanner 10.

The scanner optical path O leading from the beam discharge end of laserassembly 15 to the photoconductive surface 12 includes a first beamfolding mirror 24 mounted on end support 18 adjacent the laser output.Beam modulator 27 is disposed downstream of mirror 4 on the downwardlyinclined side support 20. Mirror 24 intercepts the laser beam 25 andturns, (i.e. folds) the beam through an angle of approximately 90° (inthe horizontal plane) and downwardly toward modulator 27. Modulator 27,which may comprise any suitable light modulator, as for example anacousto optic type modulator, selectively deflects the beam 25 toprovide zero order and first order beams 31, 32 in accordance with thevideo image signal input thereto. A beam stop 29 on side support 20intercepts the zero order beam 31. The first order beam 32 output bymodulator 27 strikes a second beam folding mirror 30 mounted on sidesupport 20 downstream of and below modulator 27.

Mirror 30 reflects the first order beam back toward laser 22 along agenerally horizontal plane paralleling base 14 to a third power mirror33. Mirror 33, which is supported on base 14 adjacent to and below laser22, folds the beam 32 back and directs the beam upwardly along a pathgenerally paralleling the surface of bridge support 19 toward polygon35. As will be seen, the scanner optical path O is such that the beamreflected by mirror 33 passes through one side of lens 45 to strike themirrored facets of scanning polygon 35.

Power mirror 33 comprises a cylindrical mirror with power in thecross-scan plane. Mirror 33 functions to aid focusing of the cross-scanbeam waist onto the facets of scanning polygon 35. Lens 45 performscross-scan focusing with the aid of power mirror 33 and collimates thebeam in the polygon facet scan direction.

Scanning polygon 35 is supported on shaft 37 of polygon drive motor 38which in turn, is suspended from the underside of bridge support 19,suitable bearing means (not shown) being provided to accommodaterotation of shaft 37 and the polygon 35 mounted thereon. Thepolygon/drive motor described preferably comprises a unitary assembly,the longitudinal axis of which, due to the mounting thereof on bridgesupport 19, is substantially perpendicular to the plane of bridgesupport 19. Inasmuch as bridge support 19 is inclined, the plane ofrotation of polygon 35 is inclined and generally parallel with the planeof bridge support 19.

Polygon 35 has a plurality of mirror-like facets 36 formed on theperiphery thereof, facets 36 reflecting the first order beam 32impinging thereon through a predetermined scan arc as polygon 35 rotatesto provide scan beam 40.

The scan beam 40 reflected by facets 36 of polygon 35 pass throughimaging lens 45, lens 45 serving to focus the beam onto thephotoconductive surface 12. Lens 45 is mounted on bridge support 19downstream of polygon 35. The now focused scan beam 40 from lens 45strikes mirror 33 which reflects the scan beam back along a planesubstantially paralleling base 14 to a fourth beam folding mirror 47.

Mirror 47, which is mounted on base 14 adjacent end section 17, reflectsthe scan beam in a generally downward direction through slot-likeaperture 49 in base 14 to the photoconductive surface 12 of theaforementioned xerographic system.

A pair of pickoff mirrors 50, 51 are mounted on base 14 in a position tointercept the scan beam 40 at the extremities of the beam sweep. Pickoffmirrors 50, 51 reflect the intercepted beam toward start of scan (SOS)and end of scan (EOS) detectors 53, 54 respectively, mounted on endsupport 18. SOS and EOS detectors 53, 54 comprise any suitable lightsensors such as photodiodes adapted to generate a signal in response tothe presence of light. The position of the cooperating pickoff mirrors50, 51 and detectors 53, 54 control the length of the line sync (LS)period.

As used herein, line sync (LS) is the period required for scan beam 40to travel from SOS detector 53 to EOS detector 54. An image line which,as will be understood, normally includes certain steady state signals orpixels before and after the image signals or pixels representing theparticular image to enable erasure of margin areas by the scan beam 40,is normally equal in period to that of the line sync (LS) signal. Wherethe image line and line sync (LS) periods are not equal, compensatingadjustments to the pixel clock frequency are automatically made toestablish equilibrium, as will appear more fully herein.

Referring to FIGS. 1-4, an adjustable support mechanism for laserassembly 15 permits servicing and/or removal and replacement of thelaser assembly in the field. Referring particularly to FIG. 4, laserassembly 15 includes laser plasma tube 22 housed in an elongatedcylindrical housing 57 having front and rear end caps 59, 60respectively attached thereto. Front end cap 59 has an aperture 61therethrough to enable the light beam 25 generated by laser 22 to passto the scanner optical path. Front end cap 59 comprises one half of thespherical bearing 58 and for this purpose, the exterior surface of endcap 59 is provided with a conical bearing surface 63, forming bearinghalf 64. The mating half 65 of spherical bearing 58 is fixedly attachedby mounting bracket 69 to end support 18 and has a spherically formedouter bearing surface 66 for mating engagement with the bearing surface63 of bearing half 64. A tapered cylindrical recess 72 in bearing half65 is coaxial with the optical axis O of scanner 15 and has a beamfocusing lens 67 disposed therein. Aperture 73 in the wall portion 74 ofthe bearing half 64 behind lens 67 permits the laser beam from lens 67to pass to the scanner optical system.

Bearing 58 serves as the front or forward support for the laser assembly15, the bearing halves 64, 65 thereof mating together to permit rotationof laser assembly 15 about a point P coincident with the optical axis Oof scanner 10 during alignment, as will appear.

Laser plasma tube 22 is fixedly disposed in housing 57 by suitable means(not shown) with the laser beam output therefrom prealigned to a presetpoint P. Point P comprises both a point on the optical axis O of ascanner 10 and the center of rotation of the sperhical bearing 58supporting the beam discharge end of the laser assembly in scanner 10.To enable point coincidence of the laser beam 25 with the optical axis Oof scanner 10 to be achieved, the laser beam is accurately prealigned tothe center of rotation of the spherical bearing 58, one half 64 of whichis part of the laser assembly 15 and the other half 65 of which is apart of scanner 10 as described above. This permits the optical axis ofscanner 10 and the axis of the laser beam to be brought into coincidenceby pivoting the laser assembly about point P following joinder ofbearing halves 64, 65.

Referring particularly to FIGS. 1-3, the opposite or rear end of housing57 is received in and supported by a rear support member 77. Supportmember 77 comprises a generally semi-circular part 78 and supportingbase 79, base 79 being fixedly secured to end support 18 as by means ofscrews (not shown). The semi-circular part 78 of support member 77 hasradially disposed screw type members 81 at predetermined spaceddistances along the periphery of segment 78, members 81 being abuttablewith the outer periphery of housing 57 to adjust the relative positionof laser assembly 15 in support member 77. As will be understood,adjustment of screws 81 displaces the laser assembly 15 relative to thesupport member 77 and effectively pivots the laser assembly about pointP when aligning the laser beam with the optical axis O of scanner 10.

A spring-like retainer 83 is pivotably secured to base 79 of supportmember 77. The opposite end of retainer 83 is adapted to lockinglyengage pin 84 proximate the end of semi-circular part 78 of supportmember 77. Retainer 83 is formed with an inwardly curved segment 85which engages the outer periphery of housing 57 on locking of retainer83 into position to bias the housing 57 into abutment with adjustingmembers 81. A radially inward projecting resilient finger 86 is appendedto retainer 83, finger 86 engaging rear end cap 60 to bias the laserassembly 15 forward and hold the bearing halves 64, 65 in matingengagement.

Referring particularly to the control schematic of FIG. 5 of thedrawings, video image signals from a suitable source (not shown) such asa raster input scanner, memory, communications channel, etc., are inputto drive module 104 of modulator 27 via serial video data line 102. Theserial stream of image signals are clocked through lead 102 to drivemodule 104 by clock signals generated by a pixel clock 105 and output,together with the line sync (LS) signal generated by SOS and EOSdetectors 53, 54, to the video data source through lines 101, 142respectfully. Polygon drive motor 38 is operated by polygon motorcircuit 107, rotation of polygon 35 scanning the first order beam 32across the photoconductive surface 12.

Video image signals are input to scanner 10 on a line by line basis,there being a pre-established number of pixels in each line. Betweenimage scan lines and at the beginning and end of each line, apredetermined steady state video signal is provided to maintain acontinuous first order beam and assure actuation of SOS and EOSdetectors 53, 54 and erasing of non-image or background areas such asthe side margins. Between the line sync (LS) signals, pixel clock 105 isinterrupted, clock 105 incorporating means to start the clocksynchronously with the laser beam's spatially sensed SOS position andthereafter gate the pixel clock off as the laser beam sweeps across EOSdetector 54. The digital line sync (LS) signal, which is derived fromthe SOS and EOS detectors, synchronizes startup and stopping of thevideo data source with scanner 10.

Referring particularly to FIG. 6, SOS detector 53 comprises a dualphotodiode detector such as a Model No. 20-10-583 detector made bySensor Technology Inc. As will be understood, each photodiode element109, 110 generates a momentary signal wave as the laser beam passesacross detector 53. The detector signals are output through lines 111,112 to comparator 114, the latter responding to the point at whichcrossover between the falling signal output of the upstream photodiodeelement 109 and the rising signal output of the downstream photodiodeelement 110 occurs to enable gate 116 of line drive circuit 118. Theoutput of circuit 118 is coupled to line sync (LS) flip-flop 120 (FIGS.5, 7a) by line 119.

The output of photodiode element 109 is fed by line 121 to a secondcomparator 124 having a preset reference signal input thereto throughline 125. The output of comparator 124 is coupled to the enable terminalof comparator 114 by line 126, comparator 124 serving to deactivate thedetection circuit in the absence of light.

The circuit for EOS detector 54 is identical to that of SOS detector 53discussed above.

To permit SOS detector 53 to be used to align laser 22 during servicingor replacement of the laser, as will appear more fully herein, theoutput of photodiode element 109 is fed through line 129 to one sectionof a dual BIFET OP AMP 130 of peak responding circuit 131. The voltagesignal level obtained is held on capacitor 134 which applies the signalto the second section of the OP AMP 130. Feedback for OP AMP 130 isderived through circuit 135.

In operation, peak responding circuit 131 provides, at the output 136thereof, a meter readable signal reflecting the peak voltage level ofthe output signal produced by the detector photodiode element 109 as thelaser beam is moved thereacross. Output 136 may be coupled to a suitablemeter such as a digital voltage meter (DVM) to permit the voltage level,which reflects the brightness of the scan beam impinging on detector 53,to be read.

EOS detector 54 may similarly be provided with peak responding circuitry131 either in place of detector 53 or in addition thereto.

Referring particularly to FIGS. 5, 7a and 7b, the SOS and EOS signalsproduced by detectors 53, 54 as the laser beam scans thereacross areinput to line sync flip-flop 120 of pixel clock 105. Additionally, theSOS signal from detector 53 is input to the set gate of end of count(EOC) flip-flop 138. The output of flip-flop 120, termed the line syncor LS signal herein, is applied through line 139 to the clock and resetgates of phase detector flip-flop pair 140, 141 of pixel clock circuit105. Additionally, the LS signal from flip-flop 120 is output to thevideo data source (not shown) through line 142 to synchronize the datasource with the scanner 10. The output of EOC flip-flop 138 is appliedthrough line 170 to the reset and clock gates of phase detectorflip-flops 140, 141, and to clock limit gate 168, and through line 172to control flip-flop 143.

A digital voltage controlled oscillator (DFVCO) 144 provides pixel clockpulses, the clock pulse output of DFVCO 144 being fed via lead 145,prescaler circuit 146, and lead 145' to clock output gate 147 (FIG. 7b).Prescaler circuit 146 serves to permit the frequency of the pixel clockpulses output by DFVCO 144 to be reduced, i.e. scaled, to the frequencydesired, circuit 146 including a frequency selector 149 to allow thescaling factor of circuit 146 to be manually set.

To enable the number of image pixels in each image line which isdeterminative of the image size, to be controlled, a divide-by-N counter150 (FIG. 7b) is provided. Counter 150 is driven by clock pulses fromthe clock output line 145' of DFVCO 144 input to counter 150 throughline 151. Counter 150 is preset to a predetermined count representingthe desired image magnification by magnification selector 152. Theoutput of counter 150 controls enabling of end of count (EOC) gate 153,the EOC signal produced by gate 153 when triggered being input throughline 155 to EOC flip-flop 138 (FIG. 7a).

Phase detector flip-flops 140, 141 serve to define the interval betweenthe EOC signal, produced by counter 150, and the falling edge of the LSsignal produced by EOS detector 54 and to adjust the output frequency ofDFVCO 144 in response thereto to provide a pixel clock frequencycompatible with both the pixel count and the period of the image line.The signal outputs of flip-flops 140, 141 are input to controller 159which outputs a control signal having a duration proportional to theinterval (if any) between the EOC and EOS signals.

The control signal of controller 159 is output through line 161 tofilter 162 which acts to both filter and to integrate the signal. Thecontrol signal from filter 162 is used to adjust, i.e. raise or lower,the signal output frequency of DFVCO 144 to maintain the EOC and EOSsignals colinear.

Pixel clock pulses are also fed to clock limit gate 168 through line169, line 169 being tapped into pixel clock line 145' for this purpose.The output of EOC flip-flop 138 is coupled to gate 168 through line 170.Clock limit gate 168 serves to prevent disabling of DFVCO 144 and thepixel clock output until the end of a clock pulse, rather than at someintermediate point in the clock pulse.

In operation the clock signal output of DFVCO 144 is set by means offrequency selector 149 of scaler circuit 146 to provide pixel clockpulses at the desired frequency. Additionally, divide-by-N counter 150(FIG. 7b) is preset by means of magnification selector 152 to thedesired number of image pixels per scanned line.

As the scan beam 40 passes across SOS detector 53, the signal generatedby detector 53 sets LS flip-flop 120 and EOC flip-flop 138. Setting ofEOC flip-flop 138 triggers gate 168 to start DFVCO 144 and initiateinput of a line of video image signals or pixels from the data source.

The pixel clock pulses generated by DFVCO 144 following scaling byscaler circuit 146, are output via line 145' to gate 147 and from gate147 and pixel clock output line 101 to the video data source (notshown). At the same time, the pixel clock pulses are fed to divide-by-Ncounter 150 through line 151.

As the imaging beam passes across EOS detector 54, the signal fromdetector 54 resets LS flip-flop 120 to terminate the LS signal, LSflip-flop 120 resetting phase detector flip-flop 141 and pulsingflip-flop 140. Counter 150 on reaching the end of the preset count forwhich counter 150 is programmed, triggers EOC gate 153 to generate anEOC signal in line 155. On the next pixel clock pulse, EOC flip-flop 138is pulsed. The signal from flip-flop 138 enables clock limit gate 168and on the next succeeding pixel clock pulse, gate 168 is triggered toinactivate pixel clock 144. The signal from flip-flop 138 resets phasedetector flip-flop 140.

Where phase detector flip-flops 140, 141 are reset at substantially thesame time, the signal inputs to controller 159 balance and no clockadjusting signal appears at the output of controller 159. Where thefalling edge of the LS signal and the EOC signal occur at differenttimes, the resulting control signal output by controller 159 advances orretards DFVCO 144 in accordance with the interval between the LS and theEOC signals to provide a corresponding adjustment in pixel clockfrequency.

Referring now to FIG. 8, polygon motor drive circuit 107 includes aquadrature oscillator 173 adapted to generate two sine wave outputs inphased quadrature, referred to herein as sine and cosine signals. Thesine and cosine signals are output through lines 174, 175 respectivelyto amplifiers 176, 177 of linear amplifier circuits 178, 179 where thesignals are amplified. The amplified sine and cosine signals are fed tothe field windings 182, 183 of the two phase polygon motor 38 toenergize windings 182, 183 and operate motor 38.

To maintain polygon speed constant, the current of motor 38 is sensedand the signal fed back by lines 184, 185 to amplifier circuits 176,177. The return signal serves to control the current output of amplifiercircuit pair 178, 179 in accordance with changes in polygon speed tomaintain the speed of polygon motor 38 and hence, polygon 35 constant.Adjustable resistor 188 permits the signal output frequency ofoscillator 173 and hence the rotational speed of polygon motor 38 andpolygon 35 to be adjusted.

Referring particularly to FIG. 9 video image signals output from thevideo data source to line 102 and modulator driver module 104 aretranslated by translator 189 and input via line 190 to OR function gate191. To accommodate shutdown of the data source (not shown) orunplugging of scanner 10, the presence of video image signals is sensedby translator pair 193, 194 via line 192. Translator pair 193, 194 arecoupled by lines 195, 195' to suitable voltage reference sources, withthe output thereof coupled through lines 196, 197 to OR function gate198. Gate 198 is coupled by line 199 to gate 191. In the event of a lossof video image signals due to either shutdown of the data source orunplugging of scanner 10, translator 193 or 194 responds and provides asteady state signal to operate driver module 104 and cause modulator 27to output a first order beam 32.

To permit test images to be input to scanner 10, a test line 202 isprovided, test line 202 being coupled to one input of OR function gate203. Line 204 feeds video image signals output by gate 191 to gate 203.Line 206 couples gate 203 to the control transistor 205 of a singlefrequency gateable oscillator 207 which serves to produce high frequencysignals in output lead 208 to modulator transducer 210 in accordancewith the video image signal content.

OPERATION

At startup of scanner 10 power is applied to quadrature oscillator 173of polygon motor drive circuit 107. The sine and cosine signal output ofoscillator 173 triggers linear amplifier pair 178, 179 (FIG. 8) in phaserelationship to energize windings 182, 183 of polygon motor 38 androtate polygon 35 at a constant speed. Laser 22 is energized, the beam25 emitted by laser 22 being reflected by mirror 24 to modulator 27. Itis understood that solenoid 23 is also energized to withdraw shutter 21by suitable circuit means (not shown).

Video image signals from the video data source are input through line102 to drive module 104 of modulator 27 in response to the line sync(LS) signals, the image signals being clocked by clock pulses output bypixel clock 105. The video image signals are amplified by amplifier 189and input via line 190, gate 191, line 204, and gate 203 to controltransistor 205 of the gateable oscillator 207 (FIG. 9). At other times,i.e. between lines, a steady state signal is applied to the modulatordrive module 104 by translator 143 or 144 via line 196 or 197, gate 198,line 199, and gate 191 (FIG. 9).

Oscillator 207 of modulator drive module 104 normally outputs a RFsignal to line 208 and transducer 210 of modulator 27. As a result,transducer 210 generates a pulsed acoustic wave within the modulatormaterial which causes a periodic change in the index of refraction ofthe modulator 27 and deflection of the laser beam 25 to provide thefirst order beam 32. The angle of deflection is dependent on theacoustical frequency and the angle between the laser beam 25 and theacoustical beam. In the absence of an acoustic wave, the laser beam 25passes through modulator 27 (zero order beam 31) and is intercepted bybeam stop 29. In that event, exposure of the previously chargedphotoconductor surface 11 to beam 25 is prevented and the charged areais developed as will be understood by those skilled in the art. Wherethe acoustic wave is present, beam 25 is deflected into the optical pathO leading to the photoreceptor 11.

In the exemplary arrangement shown, where the video signal is low (i.e."0"), the RF signal produced by oscillator 207 is output to transducer210. The acoustic waves generated by transducer 210 deflect the laserbeam to provide the first order beam 32. The beam passes through thescanner optical path and impinges on photoreceptor 11, discharging thearea struck and preventing development thereof. When the video signal ishigh (i.e. "1" ), control gate 205 is triggered to interrupt the RFsignal output of oscillator 207. This produces a zero order beam 31which passes directly through modulator 27 to beam stop 29.

Between image lines, when input of video signals is interrupted,translator 194 responds to provide a steady state video signal (i.e.,"0"). As a result, the RF signal output of oscillator 207 to transducer210 deflects the laser beam to provide first order beam 32 betweenlines.

As the beam 32 is swept across photoreceptor 11, SOS detector 53produces a signal which sets LS flip-flop 120 and EOC flip-flop 138(FIGS. 7a, 7b). Setting of LS flip-flop 120 generates a line sync (LS)signal which is output through lead 142 to enable the data source andinitiate transmission of video image signals to line 102. At the sametime, the signal from EOC flip-flop 138 enables DFVCO 144. Pixel clockpulses generated by DFVCO 144 are output via lead 145, prescaler circuit146, lead 145' and gate 147 to clock output lead 101 and the video datasource. Pixel clock pulses are also output through lead 151 todivide-by-N counter 150 which, following a preset count equal to thenumber of image signals or pixels in the image line enables EOC gate153. EOC gate 153 resets EOC flip-flop 138 to terminate operation ofDFVCO 144. Phase detector flip-flop 140 is reset at the same time.

As the image beam sweeps across EOS detector 54, the signal fromdetector 54 resets LS flip-flop 120 to terminate input of video imagesignals the video data source and reset phase detector flip-flop 141. Asdescribed earlier, where resetting of phase detectors 140, 141 occurs atdifferent intervals, a signal is output by controller 159 to eitherspeed up or slow down DFVCO 144 and correlate the output of pixel clockpulses with the sweep velocity of the beam 32.

During operation, the light beam 25 emitted by laser 22 is turnedthrough an arc of more than 90° (in the horizontal plane) and reflecteddownwardly by fold mirror 24 to the inlet of modulator 27 (FIGS. 1-3).As described, modulator 27 either directs the beam against beam stop 29(zero order beam 31) or to fold mirror 30 (first order beam 32) inaccordance with the content of the video image signals input thereto.The first order beam 32 striking mirror 30 is reflected back in thedirection of laser 22 but along a lower level of scanner 10 to powermirror 33 disposed in the area below laser 22. Mirror 33 reverses (i.e.folds) the beam 32 and turns the beam upwardly to cause the beam toimpinge against the mirror-like facets 36 of the rotating polygon 35.Polygon 35 sweeps the light beam through a preset scan arc and reverses(i.e. folds) the beam direction so that the scan beam 40 passes throughlens 45 to power mirror 33. As will be understood, lens 45 serves tofocus the scan beam 40 onto the photoconductive surface 19. As the scanbeam 40 emitted from lens 45 sweeps across mirror 33, the beam isreversed (i.e. folded) and directed forward by mirror 33 to fold mirror47. Mirror 47 turns the scan beam 40 downwardly through scan slot 49 toimpinge on the photoreceptor 11.

As the scan beam 40 is swept by polygon 35 through the scan arc, pickoffmirrors 50, 51 intercept the beam. Light reflected by mirrors 50, 51impinges on SOS and EOS detectors 53, 54 to provide the aforedescribedSOS and EOS signals.

Where replacement or off line servicing is required, the laser assembly15 is removed from scanner 10 by releasing spring retainers 83, 86 andwithdrawing the laser assembly 15. Separation occurs at the sphericalbearing halves 64, 65 (FIGS. 1-4).

When replacing or installing a new laser assembly 15, the procedure isreversed. However, following replacement, a critical alignment of thelaser beam with the scanner optical path O must be made if scanner 10 isto operate correctly.

For realignment following replacement or servicing, the laser assembly15 is positioned so that bearing half 64 of spherical bearing 58, whichis attached to laser assembly 15, is fitted over the bearing half 65fixed to the scanner frame. The cylindrical exterior of the laserhousing adjacent the opposite end thereof rests against rear supportmember 77 and the adjusting screws 81 projecting therefrom. Springretainers 83, 86 are brought into place to bias the laser assemblyagainst support member 77 and screws 81 thereof, and to force the laserassembly forward to bring bearing halves 64, 65 of spherical bearing 58into mating contact.

As described, laser plasma tube 22 of laser assembly 15 is prealignedsuch that on mounting of the laser assembly in place on scanner 10, thebeam 25 emitted by laser 22 intersects point P on the scanner opticalaxis O which is also the center of rotation of spherical bearing 58.Following mounting of the laser assembly 15 on scanner 10, laser 22 isturned on and a suitable meter, i.e. a DVM, coupled to the output leadof peak responding circuit 131. It is understood that either or bothdetectors 53, 54 may be provided with peak responding circuit 131 and inthe case where both detectors incorporate circuit 131, either or bothdetectors are used to effect alignment. Polygon motor 38 is energized torotate polygon 35.

In the absence of a video signal, a steady state signal is produced bytranslator 193 or 194 to cause the RF signal output of oscillator 173 tobe applied to transducer 210. The RF signal input to transducer 210generates an acoustic wave in the modulator material to deflect thelaser beam and provide first order beam 32 as described heretofore. Bymeasuring the output of peak responding circuit 131, which is a measureof the intensity of the laser beam, as the beam passes across thedetector, the laser assembly 15 may be incrementally adjusted by meansof screws 81 until a maximum meter reading, representing the maximumintensity, is obtained. In this process, it is understood that sphericalbearing 58 permits omnidirectional pivoting of the laser assembly 15about the point (P) until maximum beam intensity, representing axialalignment of the laser beam with the scanner optical axis is achieved.

It is understood that the aforedescribed laser supporting mechanismserves to retain, i.e. lock, the laser assembly in position once thecritical alignment has been achieved. And while a ball point type ofsupport for laser assembly 15 is illustrated herein, other support typespermitting pivoting of the laser assembly 15 about a predetermined pointmay be envisioned.

While the invention has been described with reference to the structuredisclosed, it is not confined to the details set forth, but is intendedto cover such modifications or changes as may come within the scope ofthe following claims:

I claim:
 1. In a raster scanner, the combination of:(a) a source of ahigh intensity beam of light; (b) a first mirror for intercepting saidbeam and reflecting said beam forward along a downwardly inclined slope;(c) a second mirror for intercepting the beam from said first mirror andfolding said beam so that said beam is reflected backwards along asubstantially horizontal plane; (d) a polygon for scanning said beamthrough a predetermined scan arc; and (e) a third mirror forintercepting the beam reflected from said second mirror and folding saidbeam so that said beam is reflected forward along an upwardly inclinedslope toward said polygon; (f) said polygon intercepting the beam fromsaid third mirror and reflecting said beam downwardly along said slopeback to said third mirror; (g) said third mirror intercepting said scanbeam from said polygon and reflecting said scan beam forward along saidhorizontal plane to the object to be scanned.
 2. The raster scanneraccording to claim 1 including lens means for focusing said scan beam onthe object to be scanned, said lens means being disposed astride thescan beam path between said polygon and said third mirror.
 3. The rasterscanner according to claims 1 or 2 including a modulator for modulatingsaid beam in response to image signals, said modulator being disposedastride the beam path between said first and second mirrors.
 4. Theraster scanner according to claim 1 in which the distance between saidlight source and said polygon is substantially equal to the distancebetween said polygon and the object to be scanned.
 5. In a rasterscanner the combination of:(a) a beam of high intensity light; (b)plural mirrors cooperating to form an optical path for routing saidlight beam back and forth and up and down between at least two operatinglevels before discharge toward the object to be scanned whereby toprovide a relatively compact folded optical path; said mirrorsincluding:(1) a rotatable polygon for scanning said beam through apreset scan arc; (2) a first mirror for directing said beam at an anglefrom a first operating level to a second operating level; (3) a secondmirror at said second operating level for intercepting said beam, saidsecond mirror folding said beam and reflecting said beam along saidsecond operating level; (4) a third mirror at said second operatinglevel for intercepting said beam, said third mirror folding said beamand reflecting said beam at an angle from said second operating leveltoward said first operating level;said polygon intercepting said beamand reflecting said beam toward said third mirror, said third mirrorfolding the returned beam from said polygon and reflecting said beamalong said second operating level; and (5) a fourth mirror at saidsecond operating level for intercepting said beam, said fourth mirrorreflecting said beam toward the object to be scanned; and (c) lens meansfor focusing said beam on the object to be scanned.